Printers include inkjet printers having one or more printheads used to print on print media. An inkjet printhead typically includes a vertical array of inkjet nozzles. In some designs, the vertical array is a single line array aligned perpendicular to the printhead scan direction or aligned slightly tilted from perpendicular when the nozzles in the line array are fired with a time delay as is known to those skilled in the art. In other designs, the vertical array includes two or more vertical line segments horizontally spaced apart with the nozzles in one vertical line segment fired with a time delay relative to the nozzles in another vertical line segment as can be appreciated by the artisan. In still other designs, the vertical array includes two or more horizontally spaced-apart vertical lines or line segments, wherein a nozzle of one vertical line or line segment is positioned vertically between two adjacent nozzles of another vertical line or line segment. The term “printhead” means a group of pixel printing elements capable of causing any possible character or symbol (including a single or multi-pixel character or symbol) of a single color to be printed on the print media. The term “printhead” also includes the terms “pen” and “cartridge”. A typical color inkjet printer has a black printhead and three color printheads (such as a cyan printhead, a yellow printhead, and a magenta printhead). In some designs, the three color printheads are three groups of nozzles on a single printhead block mounted to the printhead carriage. Printers having horizontally spaced-apart redundant printheads are known.
Print quality depends on the skew alignment of each printhead with respect to the printhead scan direction, on the bi-directional alignment of each printhead in the forward printhead scan direction relative to the reverse printhead scan direction, and on the horizontal and vertical alignments of one printhead relative to another printhead. A conventional method of printhead alignment includes printing a printhead alignment test pattern (having spaced-apart images) on the print media, passing a printhead-carriage-mounted optical sensor along the printhead scan direction over the alignment pattern to detect the alignment pattern, using a counter-timer to measure the time it takes the optical sensor to reach the leading and/or trailing edges of the images of the alignment pattern, calculating the positions of the images from the measured times of the counter timer, and determining the printhead misalignments from the calculated image positions. The spaced-apart images are identical blocks printed at a 100% print density for better determination of the edges of the images by the sensor.
Print quality during printing also depends on reducing paper cockle which is the wavy appearance of the print media paper due to exposure to moisture from the ink deposited thereon by the printhead during printing. It is known to reduce paper cockle by printing a page wherein each print object (such as, but not limited to, a text character or a symbol) is printed at a same print density (sometimes also called a gray scale regardless of ink color) less than 100%.
What is needed is an improved method for determining a printhead misalignment of a printer and an improved printhead alignment test pattern.